Next week I will be away at a place which is pretty remote and has no Catholic churches, so I will be asking my pastor to dispense me from the obligation to attend Mass on the Sunday and Holy Day (Assumption) that will fall while I am away. I'm not discussing that particular problem here.

What I was wondering was what form dispensations of this type usually take. Are they usually verbal, or written? I want to make sure this is done in the correct manner so as to quash any possibility of me incurring mortal sin by not attending Mass on those two days.

As you are asking your priest, you can also ask that question of him, of course.
However as God’s representative to you in your parish, his verbal permission should be sufficient.
May God bless you in your absence.
If you travel with a computer or iphone and will be in a region where internet is available it may be of some comfort to participate as well as you can manage with an online webcam Mass. That of course doesn’t replace your attendance at Mass but may be a way to share to some degree in the community spirit of worship in God’s Church

Next week I will be away at a place which is pretty remote and has no Catholic churches, so I will be asking my pastor to dispense me from the obligation to attend Mass on the Sunday and Holy Day (Assumption) that will fall while I am away. I’m not discussing that particular problem here.

What I was wondering was what form dispensations of this type usually take. Are they usually verbal, or written? I want to make sure this is done in the correct manner so as to quash any possibility of me incurring mortal sin by not attending Mass on those two days.

Thanks so much!

It is twofold, attend Mass and abstain from labors (as explained below). If there are no Catholic churches, then it is morally impossible so there is no obligation to attend Mass, but one should still abstain from labors.
Canon 1247
On Sundays and other holy days of obligation the faithful are bound to participate in the Mass; they are also to abstain from those labors and business concerns which impede the worship to be rendered to God, the joy which is proper to the Lord’s Day, or the proper relaxation of mind and body.
Canon 1248
1. The precept of participating in the Mass is satisfied by assistance at a Mass which is celebrated anywhere in a Catholic rite either on the holy day or on the evening of the preceding day.
2. If because of lack of a sacred minister or for other grave cause participation in the celebration of the Eucharist is impossible, it is specially recommended that the faithful take part in the liturgy of the word if it is celebrated in the parish church or in another sacred place according to the prescriptions of the diocesan bishop, or engage in prayer for an appropriate amount of time personally or in a family or, as occasion offers, in groups of families.